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Hw6K-G1-Periodicity - HW Set 6 KEY PERIODIC TRENDS CHM 2045...

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HW Set 6- KEY PERIODIC TRENDS CHM 2045 Hw6K-G1-Periodicity.wpd General Periodicity 1. The periodic law states that the chemical and physical properties of the elements are periodic or repeating functions of their atomic numbers. This law leads us to look for trends within a specific period or family of the periodic table for the compounds listed below by arranging them in their order of appearance within a given period and/or family. An estimate may then be made as to the missing property of a given element or compound. a) The four hydrides listed contain a central atom belonging to Group IV A. When arranged in order of increasing atomic number, a trend in BP becomes obvious. The observed BP of SnH 4 is -52 / C CH 4 , - 164 / SiH 4 , - 112 / GeH 4 , - 90 / SnH 4 , (?) b) PH 3 and SbH 3 yield family trends with AsH 3 (Group V A), and GeH 4 and H 2 Se illustrate periodic trends. The observed BP of AsH 3 is - 55 / C . PH 3 (-88 / ) GeH 4 AsH 3 H 2 Se (-90 / ) ( ? ) (-42 / ) SbH 3 (-17 / ) 2. Tellurium, Te, has a greater atomic mass (127.60 g/mole) than iodine, I (126.9045 g/mole), but iodine has a larger atomic number than tellurium (53 vs. 52). Since the modern periodic table establishes a progression of increasing atomic number, this means that Te should precede I which is the reverse of the order predicted by a progression of atomic mass for these two elements. The early chemists were not aware of the concept of atomic number, but were still able to deduce the correct placement of the two elements by comparing their respective chemical and physical properties to their neighboring elements. For example, Te forms Na 2 Te analogous with the other sodium compounds of the Group VI A (Na 2 Se, Na 2 S, Na 2 O). Iodine forms compounds similar to the elements in the group VII A (NaF, NaCl, NaBr, and NaI). Atomic or Covalent Radius 3. Trends in covalent radii often can be related to periodic trends in effective nuclear charge, Z eff . Recall that Z eff increases across a period (largely because of the increasing nuclear charge and inefficient screening by added valence electrons). Z eff decreases as elements progress down a group (increasing distance from the nucleus and greater screening by the greater number of filled inner shells). As Z eff is observed to increase, the outer valence electrons are pulled in closer to the nucleus causing shrinkage of the atomic radii. Therefore covalent radii generally decrease slightly as the elements progress across a period (L ! R), while covalent radii increase rather dramatically as the elements progress down a family. 4. a) Si > S b) Sn > Si c) Ga > Si d) Al > Si e) Mg > Si f) Si > F g) Si > C h) Sr > Sb i) K > Mg j) Ge > P k) Al > Cl l) P > O m) I > Cl
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2 5. a) Na > Mg > Be b) As > Se > S c) Ba > Au > Ag > Te > Ar > H d) Rb > K > Ca > Ti > Br > Cl Ionization Energy 6. Sc ! Sc + + e - IE 1 Sc + ! Sc 2+ + e - IE 2 Sc ! Sc 3+ + e - IE 3 7. a) Removal of the first electron leaves behind a positive ion (The number of protons exceeds the remaining number of electrons by one). The remaining electrons are now held more
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Hw6K-G1-Periodicity - HW Set 6 KEY PERIODIC TRENDS CHM 2045...

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